1. Field of the Invention
The present invention relates to a method for driving a flat panel display apparatus, and also relates to a driving circuit for the display apparatus. More particularly, the invention relates to a method for driving a display apparatus which receives a digital image signal to produce a display image with gray scales in accordance with the received digital image signals, and it also relates to a driving circuit for such a display apparatus.
2. Description of the Related Art
FIG. 1 shows a data driver exemplifying a conventional driving circuit for driving a display apparatus which receives digital image data to produce a display image with gray scales in accordance with the received data. For simplicity of explanation, it is herein assumed that the digital image data consists of two bits (D.sub.0, D.sub.1). This data driver supplies driving voltages to N pixels (where N is a positive integer) on a scanning line which has been selected by means of a scanning signal.
FIG. 2 shows a circuit constituting part of the data driver of FIG. 1. This circuit, which is denoted by the reference numeral 20, supplies a driving voltage through a data line to the "n"th pixel (where n is an integer of 1 to N) of the above-mentioned N pixels provided along the single scanning line. The circuit 20 includes sampling (primary) flip-flops 21 each for receiving one bit of the digital image data (D.sub.0, D.sub.1), holding (secondary) flip-flops 22 each also for receiving one bit, a decoder 23 and four analog switches 24 to 27. To the analog switches 24 to 27, signal voltages V.sub.0 to V.sub.3 are respectively supplied from four different voltage sources. As the sampling flip-flops 21, D flip-flops or various other flip-flops can be used.
The circuit 20 shown in FIG. 2 operates as follows. On receiving the leading edge of a sampling pulse T.sub.smpn corresponding to the "n"th pixel, the sampling flip-flops 21 obtain the digital image data (D.sub.0, D.sub.1) and hold the thus obtained data therein. When such image data sampling for the 1st to Nth pixels on a single scanning line is completed (i.e., sampling corresponding to one horizontal period is completed), an output pulse OE is applied to the holding flip-flops 22. On receiving the output pulse OE, the holding flip-flops 22 obtain the digital image data (D.sub.0, D.sub.1) from the sampling flip-flops 21, and transfer the thus obtained digital image data to the decoder 23. The decoder 23 decodes each bit of the digital image data (D.sub.0, D.sub.1), and turns on one of the analog switches 24 to 27 in accordance with the respective values of the thus decoded bits. As a result, one of the signal voltages V.sub.0 to V.sub.3 from the four different voltage sources, which corresponds to the thus turned-on analog switch 24, 25, 26 or 27, is output from the circuit 20.
A conventional data driver such as described above requires 2.sup.n different voltage sources (where n is the number of bits constituting digital image data). In other words, the number of required voltage sources doubles when the digital image data is enlarged by one bit. For example, in the case where the digital image data consists of 4 bits for the generation of a display image with 16 gray scales, the number of required voltage sources is: 2.sup.4 =16. Similarly, in the case where the digital image data consists of 5 bits for the generation of a 32-gray-scale display image, the number of required voltage sources is: 2.sup.5 =32. In the case of 6-bit digital image data for the generation of a 64-gray-scale display image, the number of required voltage sources is: 2.sup.6 =64.
Such voltage sources are connected through the analog switches of the data driver to a display apparatus, e.g., a liquid crystal panel, which provides a heavy load on the voltage sources. Thus, each voltage source is required to have a sufficient performance to drive such a heavy load. The increase in the number of such high-performance voltage sources is a significant factor in the higher production cost of the entire driving circuit. Furthermore, since high-performance voltage sources cannot readily be placed within the LSI circuit constituting the driving circuit, they must be located outside the LSI circuit. This means that signal voltages for driving the liquid crystal panel must be supplied from external voltage sources to the LSI circuit. As a result, with an increase in the number of voltage sources, the number of input terminals of the LSI circuit must be increased accordingly. It is extremely difficult to produce an LSI circuit having such a large number of input terminals. Even if it is possible to make such an LSI circuit, mounting or manufacturing problems arise in the mass production thereof; it is practically impossible to mass-produce such LSI circuits.
To solve the above-described problem, an oscillating voltage driving method and a driving circuit using this method have been proposed by Japanese Patent Application No. 4-129164, which has not been published. In the proposed method and driving circuit, external voltage sources are provided to supply gray-scale reference voltages which are used to further obtain a plurality of interpolated voltages, so that gray scales can be obtained using both the gray-scale reference voltages and the interpolated voltages. Thus, the number of gray scales which can be obtained is larger than that of the voltage sources in the driving circuit. Several types of data driver using this oscillating voltage driving method have been put into practical use.
FIG. 3 shows a circuit 30 constituting part of a data driver exemplifying the proposed driving circuit using the above-described oscillating voltage driving method. In the circuit 30, four interpolated voltages (V.sub.0 +2V.sub.2)/3, (2V.sub.2 +V.sub.5)/3, (V.sub.2 +2V.sub.5)/3 and (2V.sub.5 +V.sub.7)/3 can be obtained from four gray-scale reference voltages V.sub.0, V.sub.2, V.sub.5 and V.sub.7 which are supplied from external voltage sources. From the four gray-scale reference voltages and the four resultant interpolated voltages, eight gray scales are obtained. Thus, the provision of only four voltage sources for supplying gray-scale reference voltages makes it possible to obtain eight gray scales.
FIG. 4 shows, by way of example, the waveform of a signal voltage V.sub.1 which is output to a data line from the circuit 30 of FIG. 3, and the waveform of a signal voltage V.sub.COM applied across a common electrode (not shown) of a liquid crystal panel which is driven by this conventional data driver in accordance with a known alternating driving method. It is assumed in FIG. 4 that the entire driving circuit operates under the ideal condition of no load. The signal voltage V.sub.1 is one of the four interpolated voltages described above, which is produced from the gray-scale reference voltages V.sub.0 and V.sub.2 in the case where the value of the digital image data is 1. Voltages V.sub.1.sup.+ and V.sub.1.sup.- indicate voltages applied to a pixel in a positive period (field) and in a negative period (field), respectively. The waveforms of the gray-scale reference voltages V.sub.0 and V.sub.2 are shown in FIG. 5, for comparison with the signal voltage (interpolated voltage) V.sub.1. As shown in FIG. 4, the signal voltage V.sub.1 periodically oscillates between the two gray-scale reference voltages V.sub.0 and V.sub.2 in such a manner that the ratio of the total time for V.sub.0 to that for V.sub.2 in one output period is 1:2. A voltage such as this signal voltage V.sub.1, which periodically oscillates between two different voltages, is known as an oscillating voltage.
This conventional data driver operates in accordance with a so-called "line inversion method" in which the polarity of signal voltages is changed from positive to negative or vice versa at the beginning of each horizontal period, thereby preventing the deterioration of the liquid crystal device. One output period is usually set equal to one horizontal period.
FIG. 6 shows a power supply circuit 60 for supplying the above-mentioned gray-scale reference voltages V.sub.0 and V.sub.2 to the data driver. The power supply circuit 60 includes operational amplifiers 61 and 62. The oscillating voltage V.sub.1 in FIG. 4 can be obtained by allowing the power supply circuit 60 to alternately output the two gray-scale reference voltages V.sub.0 and V.sub.2 during one output period.
In the above-described conventional data driver using the oscillating voltage driving method, a plurality of interpolated voltages are obtained from the gray-scale reference voltages, so that a large number of gray scales can be obtained by the use of a limited number of voltage sources. This conventional data driver, however, involves such problems as will be described below.
FIG. 7 shows an equivalent circuit of a data line which is connected to the data driver and accordingly provides a load on it. In a data line actually used, capacitance and resistance exist as distributed constants. On the other hand, with a data line considered as a load, such capacitance and resistance can be considered simply as concentrated constants R.sub.S and C.sub.S. For example, the concentrated constants R.sub.S and C.sub.S may be set to 50 k.OMEGA. and 100 pF, respectively.
A single data line such as described above provides only a light load. But the number of the data lines used in a liquid crystal panel is so large that the total load provided by the data lines becomes significant. For example, in a VGA-compatible liquid crystal panel, the number of its data lines is: 640.times.3=1920. If all the values of digital image data corresponding to a single horizontal (scanning) line are 1, 1920 circuits identical to the circuit 30 of FIG. 3 output oscillating voltages V.sub.1 to the 1920 data lines respectively connected thereto. Due to the application of the oscillating voltage V.sub.1 to all the 1920 data lines, 1920 such equivalent circuits as shown in FIG. 7 function together as a load on the power supply circuit 60 of FIG. 6. In this case, if the sum of the absolute values of potential differences V.sub.1.sup.+ and V.sub.1.sup.- each between the oscillating voltage V.sub.1 and the voltage V.sub.COM applied to a common electrode shown in FIG. 4 is 10 V, the maximum current flowing through the power supply circuit 60 is: (10 V/50 k.OMEGA.).times.1920=400 (mA). Such a high-scale current flows through the power supply circuit 60 immediately after the voltage polarity has been reversed, i.e., at the beginning of one output period. Thus, the entire driving circuit is in a transient state in the initial part of one output period. In such a transient state, when the output voltage from the power supply circuit 60 is switched from one gray-scale reference voltage to another (e.g., from V.sub.0 to V.sub.2) and vice versa at a high speed to obtain an oscillating voltage (e.g., V.sub.1), parasitic oscillation is very likely to arise in the power supply circuit 60 due to the high-scale current flowing therethrough. As a result, the operation of the power supply circuit 60 is prone to be unstable.
FIG. 8 shows an example of the waveform of the gray-scale reference voltage V.sub.0 supplied from the power supply circuit 60 in which parasitic oscillation has occurred. This unnecessary parasitic oscillation causes problems such as an increase in power consumption and generation of heat in the power supply circuit 60.
A possible way to prevent such parasitic oscillation is to decrease the slewing rates of the operational amplifiers 61 and 62 of the power supply circuit 60. The decrease in the slewing rates, however, deteriorates the characteristics of the entire driving circuit, such as its current response characteristics or rise time.